1. Field of the Invention
The present invention relates to an electrolysis of an aqueous solution of an alkali metal chloride. More particularly, it relates to an electrolysis of an aqueous solution of an alkali metal chloride to produce an alkali metal hydroxide having a concentration of 30 wt.% or more and chlorine in high current efficiency and low voltage by an ion exchange membrane electrolysis using a modified cation exchange membrane of a fluorinated polymer.
2. Description of the Prior Art
Recently, it has been proposed to employ an ion exchange membrane electrolysis using an ion exchange membrane instead of a conventional asbestos diaphragm so as to produce chlorine and an alkali metal hydroxide having high purity which does not include a raw material of an alkali metal chloride component. The electrolysis has been employed in an industrial operation.
In the electrolysis, a cation exchange membrane of a fluorinated polymer has been used as the ion exchange membrane from the viewpoint of chlorine resistance and alkali resistance. It has been known that the cation exchange membrane of a fluorinated polymer having carboxylic acid ion exchange groups is especially superior, as an ion exchange membrane for an electrolysis of an alkali metal chloride, to those of ion exchange membrane having sulfonic acid groups as the ion exchange groups because an alkali metal hydroxide and chlorine can be produced in high current efficiency even if the concentration of an alkali hydroxide solution is high. (The U.S. Pat. Nos. 4,065,366 and 4,116,888)
The cation exchange membrane of a fluorinated polymer having carboxylic acid type ion exchange groups which has an ion exchange capacity of 0.8 to 2.2 meq/g. dry polymer has been reported to have good characteristics, generally.
The characteristics of the ion exchange membrane are varied and a concentration of an alkali metal hydroxide obtained in high current efficiency is varied depending upon the ion exchange capacity.
According to the inventors studies, it is found that the ion exchange membrane having a smaller ion exchange capacity is advantageous for producing alkali metal hydroxide having a lower concentration whereas the ion exchange membrane having a larger ion exchange capacity is advantageous for producing alkali metal hydroxide having a higher concentration.
Thus, an alkali metal hydroxide having a lower concentration can be produced in higher current efficiency by using an ion exchange membrane having a lower ion exchange capacity. In this case, the ion exchange capacity is low whereby an electrical resistance of the membrane is large and a cell voltage is disadvantageously higher. On the other hand, when the ion exchange membrane having a larger ion exchange capacity is used for producing an alkali metal hydroxide having a lower concentration, the current efficiency itself is disadvantageously low.
The inventors have studied to overcome said disadvantageous in the production of an alkali metakl hydroxide having a concentration of 30 wt.% or higher and to provide an electrolysis of an aqueous solution of an alkali metal chloride in high current efficiency and low voltage. As the result, the inventors have found that the purpose can be satisfied by modifying a surface of the cation exchange membrane of a fluorinated polymer.